1. Field of the Invention
This invention relates to a cathodic electrode
2. Prior Art
This invention relates to a cathodic electrode structure for sodium sulphur cells.
In a sodium sulphur cell, a solid electrolyte material separates molten sodium, forming the anode, from a sulphur/polysulphide cathodic reactant. In such a cell the solid electrolyte is a material, such as beta alumina, which conducts sodium ions. On discharge of the cell, the sodium gives up electrons at the anodic interface of the solid electrolyte. The sodium ions pass through the solid electrolyte into the cathode adjacent to the electrolyte. The electrons pass through the sodium to the anode current collector and thence around an external circuit to a cathode current collector, e.g. a carbon or graphite tube or rod, in the cathodic reactant. The electrons must pass from this cathode current collector to the region of the cathode adjacent the surface of the solid electrolyte where they react with the sulphur to form sulphide ions. The sulphide ions and sodium ions form a polysulphide. The electronic conductivity of molten sulphur is low and hence it is the practice to pack the cathodic region with a fibrous carbon or graphite material which provides the required electronic conductivity.
The present invention is concerned with modifying the wetting properties of the fibrous packing to ensure the most efficient utilisation of the reactants. When the cell is being recharged it is essential that all sodium ions in the sulphur electrode have access to the electrolyte, either by direct contact between polysulphide and electrolyte or by ionic conduction through adjacent regions of polysulphide. In practice the fibrous material acts as a capillary system and influences the distribution of reactants, and in the fibrous materials commonly used it is possible for some polysulphide to become isolated from the electrolyte surface. This results in incomplete utilisation, so that the capacity of the cell gradually decreases on successive cycles of charge and discharge.
U.S. Pat. Nos. 3,966,492 and 3,985,575 disclose arrangements in which part of the cathodic region is filled with a graphite film matrix and part by a material which is more easily wetted by polysulphides; such structures however do not solve the problem of pockets of polysulphides collecting in regions remote from the electrolyte, i.e. when the greatest problem arises.